The present invention relates to an electronic switching device for use with inductive, capacitive or optoelectronic proximity switches or flow indicators. Such proximity switches or flow indicators normally incorporate an externally-influenced presence detector (comprising an oscillator with an associated switching amplifier downstream from the oscillator), an electronic switch such as a transistor, thyristor, or triac controllable from the presence detector by the switching amplifier, and a full-wave bridge rectifier for providing smooth power input to the presence detector circuit. The electronic switch of the present invention can be a two-terminal switching device (one which both receives power and transmits its detection indication over a single pair of wires). In other embodiments, the circuit of the present invention may be a three-wire switching device with two supply connections and with one additional signal output.
The electronic switches selected for use in these circuits are typically solid-state devices rather than mechanical, contact-type switching devices. Solid-state devices are especially preferred in electrical and electronic measuring, and open- and closed-loop control circuits. Solid-state devices are especially preferred for use in proximity switches. Proximity switches change state when an influencing element, to which the corresponding proximity switch is sensitive, approaches the proximity switch. When the influencing element to which the corresponding proximity switch is sensitive closely approaches the proximity switch, the presence detector circuit in the proximity switch reverses the electronic switch. In the case of a normally-open switch, the previously nonconductive electronic switch then becomes conductive; conversely, normally-closed switches will become non-conductive in response to the approach of the influencing element. With switching devices of the type being discussed it is also possible to indicate whether physical magnitude of an influencing medium, to which the switching device is sensitive, exceeds or falls below the corresponding value. Thus, an essential component of electronic switching devices of the above-described type is the externally-influenced presence detector.
The presence detector may incorporate an inductively or capacitively influenceable oscillator, so as to form an inductive or capacitive proximity switch, respectively. A photoresistor, photodiode or phototransistor can also be used as the presence detector to produce an optoelectronic proximity switch. Finally, to construct a flow indicator, a temperature measuring circuit is used as the presence detector.
In these proximity switches, K.times.V=i where K=feedback factor and V =amplification factor of the oscillator. In an inductive proximity switch, when a metal part has not yet approached to a predetermined distance, the oscillator oscillates. Once the corresponding metal part reaches the predetermined distance, the increasing attenuation of the oscillator leads to a reduction of the amplification factor V, and the amplitude of the oscillator swing lessens or the oscillator ceases to oscillate. In capacitive proximity switches, while the capacitance induced by a response body between a response electrode and a counterelectrode is low, K.times.V is less than 1, and the oscillator does not oscillate. If the response body reaches the predetermined distance, the increasing capacitance generated between the response electrode and the counterelectrode leads to an increase of feedback factor K, so that K.times.V 1, i.e., the oscillator begins to oscillate. In the inductive proximity switch and capacitive proximity switch, the electronic switch, such as a transistor, a thyristor or triac is controlled as a function of the different states of the oscillator.
Optoelectronic proximity switches have a optotransmitter and an optoreceiver and are identified as light barriers. In one type of optoelectronic switch, the optotransmitter and optoreceiver are placed on opposite sides of a monitored sector, and in another type the optotransmitter and optoreceiver are placed at the same end of a monitored sector, while a reflector placed at the other end of the monitored sector reflects the light beam from the optotransmitter to the optoreceiver. In both cases, the presence detector responds if the light beam normally reaching the optoreceiver from the optotransmitter is interrupted by an influencing element that has come into the monitored sector. There are also devices of the second type in which the light beam coming from the optotransmitter is reflected to the optoreceiver by the influencing element rather than by a fixed reflector.
Electronic switching devices of the type being discussed--like electric, mechanically operated switching devices--are needed in two types: normally open and normally closed. To minimize inventory requirements, it is not desirable to provide different devices to serve these two circuit functions. Thus, many electronic switching devices of the type being discussed can be used both as normally open and as normally closed devices. In particular, there are electronic switching devices which are "connection programmable," as disclosed in German Offenlegungschrifts 31 23 828 and 32 14 836. "Connection programmable" means that the normally-closed or normally-open characteristic of the device is determined automatically by its connection to the supply voltage.
Different types of "connection programmable" electronic switching devices are known. In one type, instead of the minimum two supply connections, three or four supply connections are provided. Depending on whether the supply voltage is connected to the first and the third supply connection (as illustrated in FIGS. 1 and 3 of DE-OS 31 23 828, and FIG. 2 of DE-OS 32 14 836), or to the first and second supply connection or to the third and the fourth supply connection (as illustrated in FIG. 4 of DE-OS 31 23 828), the switching device works as a normally open or normally closed device. Another type of device, sold in European markets under the designation "Quadronorm," has only two supply connections present. Depending on whether the positive supply voltage is applied to the first and the negative supply voltage is applied to the second supply connection or the negative supply voltage is applied to the first supply connection and the positive supply voltage is applied to the second supply connection, the "Quadronorm" switching device operates as a normally-open or normally-closed switching device.
In the "connection programmable" switching devices described above in detail--as part of the switching amplifier or between the switching amplifier and the electronic switch--an exclusive OR gate is generally needed to implement the desired operation. Commercial integrated circuits (IC's) which implement circuits of this type generally do not include an exclusive OR gate, and it must be provided in a separate circuit.
There are many integrated circuits developed especially for electronic switching devices of the type being discussed here, in which the switching amplifier has two complementary control outputs. For example, model numbers TCA 205 and TCA 305 made by Siemens and model number TDE 0160 made by Thomson-CSF have these characteristics.